Spark 4.2: SQL Last Attempt Metric (SLAM) not propagated through Comet operators

Spark 4.2 adds a "SQL Last Attempt Metric" (SLAM) feature and test suites that assert on the metric value recorded for a query's executed plan.

When Comet is enabled, Comet replaces the Spark operators with native equivalents and the SLAM metric value is not propagated through the plan, so `lastAttemptValueForDataset` / `lastAttemptValueForHighestRDDId` return `None`.

Affected Spark SQL test suites (currently marked `IgnoreComet` / `IgnoreCometSuite` in `dev/diffs/4.2.0.diff`):
- `SQLLastAttemptMetricPlanShapesSuite` (whole suite)
- `MetricsFailureInjectionSuite` (whole suite)
- `SQLLastAttemptMetricIntegrationSuite` ("dataset updates")

Start with dev/diffs/4.2.0.diff and the ignored SQLLastAttemptMetricPlanShapesSuite, MetricsFailureInjectionSuite, and SQLLastAttemptMetricIntegrationSuite dataset-updates tests. Trace how Comet’s replacement operators handle the SLAM value, then restore propagation so these suites can run without IgnoreComet/IgnoreCometSuite and their metric assertions pass.

Written by the indexing model from the issue text.